Abandoned mine sites such as this one on Galena Creek in the Barker mining district in central Montana can be the source of toxic metals in streams. Galena Creek is a tributary to Dry Fork Belt Creek and is the source of the metals at the site where scientists conducted fish toxicity experiments on Dry Fork Belt Creek.
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Fish exposed to daily changes in levels of dissolved toxic metals in mining-contaminated streams showed significantly lower mortality rates than fish exposed to constant average concentrations. U.S. Geological Survey (USGS) scientists and their colleagues, reported these finding in Environmental Toxicology and Chemistry. The lower mortality rate found in streams with cycling concentrations is an important finding considering that water-quality criteria for streams are based on constant concentrations.

Are Water-Quality Criteria Protective?

Elevated concentrations of metals in streams can be toxic to fish and other aquatic life. To protect aquatic life, agencies such as the U.S. Environmental Protection Agency have established water-quality criteria. These criteria usually are based on how well test organisms survive in laboratory experiments in which organisms are exposed to specific but constant metal concentrations. These laboratory tests do not replicate all the conditions typically found in streams, and therefore may indicate different toxicity when compared to toxicity in actual streams. Many contaminated and uncontaminated streams undergo daily variations that are caused by daily changes in temperature, sunlight, and other factors. The large daily change (diel) in metal concentrations observed in streams is one of the natural processes not accounted for in standard laboratory toxicity tests. USGS scientists and their colleagues conducted toxicity tests in two Montana streams to compare the survival of fish that were exposed to dissolved metal concentrations that were allowed to vary (diel changes) or were held constant.

Based on this initial study, current water-quality criteria appear to be protective for streams with diel concentration cycles of cadmium and zinc. Environmental regulators and water-resource managers can potentially use this information to develop sound policies regarding stream water-quality criteria, fisheries, and monitoring programs.